LIBRARYKilling EEG Noise: 50/60 Hz & the Right-Leg Drive
Mains hum is the hardest real problem in EEG. How common-mode rejection, the driven-right-leg/bias loop, and low electrode impedance beat it, and why a notch filter is a band-aid.
The EEG signal is tiny: tens of microvolts, dropping to a few µV. The 50/60 Hz hum from mains wiring is not. The body and the electrode leads act as antennas, capacitively coupling to the power line, and the resulting common-mode voltage on the body can run from a few millivolts to a couple hundred, hundreds to thousands of times larger than the brain signal you're after. Win this fight and EEG works; lose it and you record the power grid.
Common-mode rejection and CMRR
The first line of defense is the differential amplifier: it amplifies the difference between two electrodes and rejects whatever is common to both, and the mains hum is (mostly) common to both. How well it does this is its common-mode rejection ratio (CMRR); good EEG amplifiers hit 100–110 dB. But CMRR alone isn't enough, and here's the subtle part: if the two electrodes have unequal skin impedance, the common-mode voltage divides differently across each input and shows up as a difference, converting rejected noise back into signal the amplifier faithfully amplifies. Imbalanced electrode impedance, not the chip, is usually the real limit.
The (driven-right-leg, the EEG bias loop) is an active negative-feedback circuit. It senses the body's common-mode voltage (roughly the average of the inputs), inverts and amplifies it, and drives that opposing signal back into the body through a bias electrode, actively pushing the body's common-mode toward the amplifier's reference and cancelling the mains hum at its source (Winter & Webster, 1983). It buys far more rejection (tens of dB) than passive grounding. The name comes from ECG, where the unused right-leg electrode was repurposed to carry the drive; in EEG a bias electrode plays the same role, separate from the reference.
EEG NOISE · RIGHT-LEG DRIVE
Cancel the hum at the source
Body
picks up hum
Sense
the average
Invert
the opposite
Bias drive
back into body
the opposing voltage nulls the common-mode hum back at the body
series resistor limits the drive current · safety + noise, together
The bias/RLD feedback path always includes a large series resistor so that very little current can ever flow from the drive amplifier into the body; the noise-cancellation loop is designed to be current-limited. Noise performance and patient safety are solved together, not separately.
How to beat mains noise, in order
- High, balanced common-mode rejection: a good instrumentation amp AND matched electrode impedances.
- A driven-bias / right-leg loop to actively null the common-mode mains voltage.
- Low, STABLE electrode-skin impedance (prep the skin): a steady 50 kΩ beats a fluctuating 10 kΩ.
- Short, twisted leads and a driven shield to cut capacitive pickup and cable artifact.
- Distance from mains-powered equipment and power cables.
A notch filter at the line frequency attenuates EVERYTHING there, including any real brain activity at 50/60 Hz, and adds phase distortion and ringing near the notch. It's cleanup of last resort, not the fix. The real fix is good acquisition: high, balanced CMRR + a driven-bias loop + low impedance remove the hum at the source, before it's ever digitized. Reach for the notch only after you've done those.
▸Deep dive· Go deeper: gotchas and the regional 50-vs-60 split
Mains is 50 Hz across much of the world and 60 Hz in North America, and interference shows up at harmonics too (100/120, 150/180 Hz), so any line-noise strategy has to match the local grid and its harmonics. Other classic traps: forgetting the bias electrode entirely (the amplifiers aren't centered and noise dominates); the skin's own ~10–70 mV DC potential, which dwarfs the EEG, so any electrode movement injects a big artifact; and an over-aggressive bias loop, which can become unstable and actually increase interference. Stable beats heroic.
References
Keep going
Designing the front-end where all of this lives, the CMRR, the bias loop, the grounding and shielding, is the build in the OTD Academy EEG front-end project.
One Thousand Drones Academy · reviewed June 2026
Coming soon
8-Channel EEG Front-End on ESP32 →Design the analog board that reads real brainwaves: the BCI.